Production of potassium manganates



M. B. CARUS El'AL 2,940,822 PRODUCTION OF POTASSIUM MANGANATES Filed Oct. 10, 1956 June 14, 1960 MN 0 Aq. KOH

H O 0 0a VA Ol O2 KSMNO. KOH Sol'n. l I

I l 3 Aq; KOH Fiitrate 5;!40 KZMNO4 A KZMNO Slurry HEAT in KOH cl'n. F

KaMNO4, Produci:

INVENTORS.

MILTON B. CARUS ARNO H. REIDIES ATT'YS appearon the left of the equation. The

Patented June 14, 1-960 fiice PRODUCTION or POTASSIUM MANGANATES Milton B. Cams and Arno H. Reidies, La Salle, 111., as-

signors to-Carus Chemical Company, La Salle, 112., a corporation of Illinois Filed Oct. 10, 1956, Serial No. 615,032

11 Claims. (Cl. 23-58} This invention relates to the production of potassium manganates by oxidation of manganese compounds in an aqueous potassium hydroxide melt. More particularly, the invention relates to the production of potassium manganate (V), K MnO',, and to the production of potassium manganate (VI), K MnO by oxidation commencing with an oxidic manganese compound. The invention provides a process which renders production in such manner commercially practicable.

Prior to the invention, potassium manganate, K MnO has been produced for many years, and several methods of production have been devised. A large part of. the potassium manganate produced is subsequently converted to potassium permanganate. Fundamentally, the reactions involved inthe prior production of potassium manganate and potassium permanganate are represented by the following equations:

The present invention is concerned with the production of K MnO according to difierent equations, which also involve the'production of K MnO The latter compound may be recovered for use as such or forproduction of K' MnO therefrom. a

Of the several methods proposed for manufacturing K MnO the primary commercial method apparently is the roastingmethod. This involves mixing hot concentratedpotassium hydroxide and manganese dioxide, cooling and grinding the mixture. The concentrated potassium hydroxide starting material is produced by evaporating water from aqueous potassium hydroxide up to 385 C. Alternatively, a slurry of 50% KOH and manganese dioxide is sprayed into a hot oven, and the product is cooled and ground. The ground product is roasted at about 225 C. with air, while intermittently spraying water on the mixture. The roasting is carried out in large rotary drums or tubes. This operation requires a very large amount of equipment, with accompanying high capital investment, power, heat, labor and maintenance requirements. Furthermore, the process is slow, inconsistent and difiicult to supervise. The reaction cannot be carried to completion. This is apparently because of the dificul-ty in supplying suificient water to the reactants and because sufii'cient potassium hydroxide cannot be supplied. As regards the former condition, the presence of water'is necessary for the reaction although .it does not potassium hydroxide quantity, "is limited, because over, a certain ratio to manganese dioxide the product agglomerates seriously and. prevents furtheroxidation.

A number of years ago, a process was devised wherein manganese dioxide was oxidized to K MnO with air in a concentrated aqueous potassium hydroxide melt. Despite the potential attractiveness ofsuch a process, ithas apparently never been successfully employed on a combelow 5.

2 mercial scale. This is very likely due to thefact that during the process, the mixture gets very thick or viscous, so that it is extremely diificult to disperse the oxygen sufliciently for the reaction and a very large provision of power is required in order-to agitate the reaction mass at all. This thickening occurs within a" period of several hours, even with a much reduced quantity of manganese dioxide in the melt. 7

We havenow provided a process which overcomes'the foregoing difficulty of the priormelt oxidation process, and in doing so, have discovered the reasons forth'e difiiculty and have provided a solution'to-the problems.

We have found that, apparently; the principa'lsourc'e of trouble is that the manganese; dioxide swells up to many times'its original volume after a short period aroma. Even a small excess of manganese dioxide will swell'up to convert the already somewhat viscous liquid into a thick paste. a

We have discovered that-the key'to-the solution of the problem is to add a manganese oxide to an aqueous potassium hydroxide melt at a rateof addition which is not substantially greater thanits rate of oxidation toa valence above 4. in the new process, an oxidic manganese compound having a manganese valence of less than 5 is oxidized to K M'nO wherein manganese has a'valence of 5, the oxidic manganese compound being added at a rate not substantially greater than its'rate ofoxidation 10 lisrvlno I i I The K MnO produced is further oxidized to K MnOQ. The-further oxidation may takeplace in thesame reaction zone or vessel, concomitantly with or subsequent to-the oxidation of the manganese compound having'a valence Preferably, the oxidation of K Mn O to K Mn0 is' carried out in a separatenreaction zone or vessel, after removing the K MnO frointhe firstreaction zone, preferably at a rate comparabl'e'to its productiom" In prior. chemical studies,- K5Mn0ghad been produced by the following'reaction carried out with powderedniaterials in the dry state at about"800-'C..

The compound had also been producedrby a temperature decomposition ofiK Mn04, asfollowsr Both ofthese methods are technically impractical-and result in an anhydrous product.

The present invention. is: based upon the: following,

reactions: f

(s) northeastog -n ofizxm oi axon for that purpose and removing,the,.KgMnO;,;which crystallizes, as it is produced. .Reaction 6 may, take place while Reaction Sloceurs with. the continued addition of the manganese oxide. Alternatively, Reaction S' may' be 5 and16 in separate zones.

.provides- -:a practical sample, pyrolusite containing M-n O manganese 7 'completed'without substantial production of K Mn ac .cordingto Reaction ,6, and'thereafter, be oxidized to K MnO according to Reaction 6.

the K MnO may It is preferred in the invention to withdraw the K MnO ;-produced in Reaction 5 at a'rate comparable 'to or and.

approximately the same as its rate of production, oxidize it in a separate second zone according to Reac- In this manner, Reaction 6 and the KgMnQ;

4 manganeseox ide is other than manganese dioxide, and the potassium hydroxide and oxygen requirements will be adjusted accordingly.

The reaction proceeds with vigorous agitation while 5 intimately mixing an oxygen-containing gas with'the product are not complicated by thepresence of significant oxide, the: starting material in When carrying out both reactions in one manganese oxide is i found inthe K MnOl, crystalline product, and such is not the case when the reactions are carried out .in separate melt. Air is the preferred oxygen-containing gas,for economic reasons, but oxygen, air enriched with oxygen, or a mixture of oxygen and an inert gas might be'employed. The oxygen-containing gas ispreferably under 0 substantially atmospheric pressure plus any differential required to overcome theresistance to gas how, but subatmospheric or superatmospheric pressures can be provided with appropriate adjustment of the conditions of temperature and potassium hydroxide concentration.

When air is employed, it'is preferred to contact the melt Further advantages -'accrue from carrying out Reactions K MnO is rapidly, produced separatelyand in high yield, and it may be used for nther-purposes, for example,-as an oxidizing agent. Each reaction is individualy controlled, so that the manganese oxide is convertedtoIQMnO; in high yield and in a successful continuous commercial *processj Also, Reaction 6 is more favorable when carried out at a lower temperature than Reaction 5, the temperature furnishing the optimum reaction rate preferably being about 60 oxidation of the manganese toa valence of 5. The oxide with a quantity equivalent to'four or more times the theoretically required quantity of oxy'gen.

The manganese oxide is added at a rate not substantially greater than the rate of oxidation of K3MnO or may be added intermittently or continuously care being taken that at no time is a considerable'quantity of unreacted MnO present. The rate at which the manganese oxide should be'added 'is best determined empirically,

2 5 ,for each set of conditions. The examples which follow degrees lower. ,The K MnO solution produced in the v first zone may be'cooled by. evaporation .in the second zoneforthis purpose. The potassium hydroxide concentration in the second sponding' to the lower temperature.)

In certain of its broader aspects, the invention involves, then, an improvem'entin the productionof K MnQ by V oxidizing a manganese oxide in an aqueous potassium Jhydroxide melt. I The improvement comprises adding the mang'aneseo 'de to the melt and oxidizing it therein to K MnO andiproviding a rate. of manganese oxide addition-which is not substantially greater than its'rate ofoxidation to K MnO 1 The K MnO4' is advantageously oxidized to K MnO preferablyin a separate reaction. 7 zone; fIhe iinven'on thus 7 portant advantages in. the production of K MnO and i process for the production of K MnO The prior problems are overcomeyso -that it V nolonger necessary to carry out the much more cumf @bersome and 'expensi ve production of -K MnO by roasting. j Also the rate of production per unit volume is "Gmany'times greatern 'p j 1 The invention contemplates the oxidation .of an oxidic.

provides 1 commercially immangan'ese compound yvherein the manganese has a valence of less than 5; '.Various manganese oxides having amanganese valence of from'2 to 4, alone .or' incombination, pure or impure, may be employed. For ex with alkali metals maylbe employed.

' action rate is greater at V corrosion'is reduced fat ,themlovyer temperatures. The 65 some of to :1]or' greate'r,

Reaction} is carried out in an aqueous potassium 350 ,C., preferably :abo t' 240 C. 'to' 300 C. The rethe higher' temperatures, while potassium' hydroxide concentration is preferably about %Lto'90% by'weighgandpreferablyabout to ,tration andreactiontemperature maybe varied. I 4

V I f It'is also prefe ed thatthe nieltfcontain' an excess. of

potassium hydroxide over-the theoretical molar ratio" to "manganesedioxide of- 3.:-l; -j' A considerable'molar'excess on the order of 87% 0f 7 which is approximately zone may then be lower correare illustrative of substantially maximumrates of addition. The reaction mixture also can be analyzed from time to'time for K MnO content and for-MnO content, to. determine the rate of oxidatiomlfor adjusting the rate of. manganese oxide addition.

60 erably carried out at hydroxide melt at a'temperaturecf about 170? C. to

% at the temperatures] However,fthe concen- V The reactions can be carried outinbatch, semi-continuous or continuous operation. For continuous type operation, it is of course necessaryto replace the potassium lhydroxide consumed in Reaction 5.. This may be done 5 at least in part by cycling the potassiumhydroxide produced in the second zone by Reaction 6 and potassium hydroxide added to the second zone and subsequently withdrawn in more concentrated form, where the reactions are carried out in separate zones, The balance 0 of the potassium" hydroxide requirement maybe added intermittently or continuously in the'form of aqueous potassium hydroxide solutio'mwhich advantageously con tains. about 50%. R01 1, by weight, or greater. An 'equilibriumis reached in the melt bylremoval of water by evaporation, so that'the, KOH concentration, is readily maintained between 65% and 90%; i

in the same vessel, the'latter beingwith'drawn fromtime to time as it crystallizes out'of solution. The K' MnO is very soluble. and remains-in solution. It'is preferred at a more favorable. lower temperature, being cooled 5 by the evaporation of Water thereinj v In the second reaction zone, it is again preferredthat the melt contain :about 65 to 90% of potassium hydroxide, by weight. The reaction may be] carried out above the solidification point of the; mixture; andis prefabout ,140" C. to 3109C. Higher temperatures are preferably avoided," to avoid possible decomposition of K MnO It is further preferred that themelt be maintained at a temperature of. about 210 C. to 2305C. .The melt is vigorously agitated, and air or other oxygen-containing 'igas' is intimately mixed therewith, to providelabout 4 orimor 'rne's' the theoretical amountof oxygen. i 1 7 The. K MnQ 011mm an t mittently or continuously introdu edfintoi the fmeltf it is 1 preferred -to maintainia' high 'cdncentrat ion 10f K li/1110 of potassium'hydroxidej is preferably employed, on; the

which providesa" melt'of in the melt, on the order ofaboufZOO to' 400 grams per 1iter,'f or Tex-ample} The K MnO is fv'ery solublajjandis therefore ,readily oxidized to" KgMnO; in are, homo geneon's solution, morereadily the "caseiof'a 75 heterogen ou mix ure; s is the 3156 when the further oxidation is carried out concomitantly and in the same zone with Reaction 5.

The K MnO is relatively insoluble and precipitates out of the reaction mixture. It can be separated by conventional methods, such as settling and decantation, centrifugation, or filtration. The reaction can be carried out in batch, semi-continuous or continuous operation. it is preferably carried out in a continuous manner, supplying K MnO solution to the reaction zone and Withdrawing K h/inO crystals continuously or periodically as they are produced. Aqueous potassium hydroxide solution of at least 50% KOH is added to the second zone to replace the KOH withdrawn with the K MnO Carrying out the process in successive steps in the foregoing manner, conversion to K MnO and to K MnO approaches Lhe quantitative. The product of the first reaction may be, for example, an aqueous potassium hydroxide solution containing about 350-400 grams of K MnO per liter. The product of Reaction 6 initially contains adherent mother liquor, containing K MnO potassium hydroxide and water. After washing, the crystals contain about 80% to 90% of K MnO potassium hydroxide, water, and minor impurities. The K MnO is removed by washing and returned to the process, so that the yield of K MnO is practically quantitative.

Reactions 5 and 6 may be carried out in conventional apparatus, such as a vat or vats equipped with suitable means of agitation and means for dispersing air throughout the batch. Provision is made for introducing the reagents and for withdrawing the products. In the preferred process, K l n0 and K MnO are produced continuously or semi-continuously, in the manner illustrated schematically in the attached drawing.

An aqueous potassium hydroxide melt is providedin a vessel 1, and an initial quantity of the manganese oxide is added to the melt. Agitation, aeration and heat are provided, and the manganese oxide is added from time to time or continuously. Make-up aqueous potassium hydroxide is also added, according to its consumption. Bathe means schematically illustrated by abafie 2 is provided in the vessel 1, between the inlets and outlets for the materials entering and leaving the vessel.

As the production of K MnO proceeds, the solution may be allowed to overflow through a conduit Sinto 'a second reaction vessel 4, thus favorably affecting the reaction equilibrium in the first vessel 1, and supplying the initial material for the oxidation in the second vessel 4. The second vessel 4 likewise contains an aqueous potassium hydroxide melt and is supplied with means for introduction of air and for agitation.

Aqueous potassium hydroxide solution of 50% concentration or greater is supplied to the second vessel 4 from time to time, to replace material withdrawn, and it becomes concentrated by evaporation. K MnO crystallizes in the second vessel and may be withdrawn in the form of a slurry and filtered by a filter 5. The filtrate is cycled to vessel 1 through a conduit 6, to supply the potassium hydroxide requirements therein and return 14 M110, to the process. Filter cake washes preferably are used as makeup KOH solution supplied to the second vessel 4.

The manganese oxide in the first vessel is substantially prevented from entering the second vessel 4 by the baflie 2 and also owing to a relatively small flow rate. The rate of how is such ,that the. average residence or retention time for each particle is greater than the reaction time for the manganese oxide. The average residence time may be, for example, from 1 to 3 hours, while the oxidation of the manganese oxide may require about 15 minutes, as subsequently exemplified. Consequently, at most a very small amount of the oxide reaches the second vessel 4. In the second vessel, the average residence time may be about 1 to 2 hours, during which the small quantity of manganese oxide may be oxidized. The result is that very little if any unreacted manganese oxide will appear in the K MnO product.

Air-advantage of the process is that the heat requirements may be supplied entirely to one vessel, the first vessel 1. 1 The initial potassium hydroxide melt for both vessels may be prepared in the first vessel, by evaporating water from 50% KOH solution. As the reactions proceed, heat is supplied to the first vessel, furnishing the heat required to maintain the reaction temperature and concentrating the solution somewhat, by evaporation of water. The hot K MnO solution conducted from the first vessel 1 to the second vessel 4 furnishes the heat required to maintain the reaction temperature in the latter. Also, the makeup KOH solution to the second vessel is concentrated thereby, and this removal of heat cools the melt to the preferred lower temperature for Reaction 6 in the second vessel.

A number oil-additional advantages flow from the new process, which is now eminently suited for commercial operation. The process can be carried out in batch or continuous type operation, in either one or two vats,

to produce large quantities of K MnO and K MnO The simple and inexpensive equipment is substituted for an entire series of roasters, having the aforementioned disadvantages. The products are obtained in a state of high purity. The dust problem of roasting is eliminated. Operational time is greatly reduced and a much more efi'icient process is provided. The requirements of air or other oxygen-containing gas are considerably reduced, and an accompanying advantage is that correspondingly less carbonate is formed by absorption of carbon dioxide frorn the air. The air dispersion is much more intimate, and the reaction rates are improved correspondingly. Each of the melts stirs very well, so that the power requirement is low, as is the wear on the stirring apparatus.

A further advantage of considerable practical importance is that when the-process is carried out in two reaction zones, the second stage is at a lower temperature and may operate at a lower potassium hydroxide concentration. The solids and solutions are considerably easier to handle and process, and the problems of construction, corrosion, maintenance and operation of the vessels, filters, pumps and other equipment are materially reduced."

The following examples are furnished to assist in providing a complete understanding of the invention, but it is to he understood that the invention is not limited thereto nor to the quantities, materials, conditions and rocedures illustrated therein, which are merely illustrative.

Example 1 Aqueous potassium hydroxide is evaporated to prodate KOH having a volume of 4 liters. The solu tion is maintained at 300 C., and 50 grams of pyrolusite (87% MnO are added every 15 minutes, for 3 hours, or at an average addition rate of about 3 /3 grams per minute. This amounts to about of a gram per liter of reaction mixture per minute, about the maximum rate of oxidation under the conditions.

The melt is agitated vigorously, and in excess of four times the theoretical quantity of oxygen in the form of air is intimately mixed therewith. Intimate mixture is obtained by violent agitation while introducing air over the mixture, so as to intimately disperse fine bubbles of air in the suspension. Alternatively, air is introduced through a tube directly into the mixture.

The product of the oxidation is an aqueous potassium hydroxide solution containing about 15% of K MnO by weight, in a yield of approximately 98%, based on MnO about 15 minutes after the last portion of pyrolusite has been added.

Example 2.

liters of an 80% potassium hydroxide solution containing 300 grams of K MnO per liter are placed in a :thetemperature is maintained at 180 ofclectrolytic manganese dioxide are may be prepared according to Example l, or according to the succeeding examples. 'The solution or melt is maintained at a teinperature of 225 C.

, The mixture is agitated vigorously, and ,in excess times the theoretical quantity of oxygen (air) .is "intimately mixed therewith, in thefmanner of Example 1.

i' Oxidation proceeds at'the rate of about 2 grams of K 1 4:20 produced to K M nO K MnO per liter per minute. The K MnO consumed is continuously replaced, as with the solution from Example l. K MnO crystallizes out of-the reaction mixture, is withdrawn from'tirne to time in the 'form of a slurry in potassium hydroxidesolution, and is separated from the mother liquor by filtration. Makeup potassium hydroxide solution of 50%"strength or greater is added to the vessel.

Thefilter cake of K MnO is washed with 60%. aqueous fpotas'sium hydroxide solution at 100 C., and the liquid is removed from the cake by suction. The filtrate is used in the method of Example. 1 for the production of "iK3MnO4, and thewashes are employed as the above makeup solution.

The product after washing and application of suction to'remove the liquor contains 82% to 90% of K MnO some potassium hydroxide, water, andminor impurities.

. IThe yield based on K Mno 'ispi-actica'lly quantitative.

. 7 Example 3 v The' proicedure ofjExamplel is repeated, except'that C., andsl0 grams added to the mixture every 15 minutesfor 10 hours.' K MnO is pro- 'duced in a yield of about 98%, minutes after the last portion of manganese dioxide is added. I

Example' 4 1 P l-Example l isrepeated, except that 35 grams of calcined 'rh odochrosite are added every 15 minutes for 3 hours,- -insteacl of pyrolusite. The yield of K MnO is about 9 V C 7 a 7 Example 5 50 gallons of 80% aqueouspotassium-hydroxide solution ismaintained at 300 C. In the manner of Example 1, 5 pounds of'the pyrolusite are added every 15 minutes for} hours, with agitationand aerationJ The'yield'of' K MnO is about 98% V The invention thus provides a newjprocess for producing K MnO and for producing K 'MnO which constitutes a substantial improvement over the prior methods and overcomes their disadvantages. A major characteris- The invention is hereby claimed as follows: 7 V

1. In a process for producing 'K MnQgby'oxidizing a manganese oxide having a manganese valence of less than 5 in a hot aqueous potassium hydroxide solution, the

improvement which comprises adding said manganese oxide to said solution and oxidizing it therein to K MnO zone.

the average rate of said manganese oxide addition being a maximum of about of a gram per liter of reaction mixture per minute and being not substantially greater than its rateof oxidation'to K MnO ,'and oxidizing the gas'with said solution to oxidize said manganese oxide to i K MnO the rate of said manganese oxide addition being not substantially greater than ,its rate of oxidation to K MnO removing the K MnO produced, and oxidizing the K MnO removed to K MnO in a separate reaction 3. In a process for producing K 'MnO by oxidizing a manganese oxide having a manganese valence of less than 5 in a hot aqueous potassium hydroxide solution, the improvement which comprises providing an aqueous about 65% to 90% potassium hydroxide solution and maintaining it at atemperatureof about 240 C. to'300 0, adding said manganese ,oxide to said solution, intimately mixing an oxygen-containing gas with'said solution to oxidize said manganese oxide to K MnO the rate of said manganese .oxide'addition being not substantially greater than its rate-of oxidation to K MnO removing 'the K MnO produced, and oxidizing the K Mn'O removed to' K MnO inan aqueous about 65% .to 90% potassium hydroxide solution maintained at a temperature of about 210 C. to 230 C. in, a separate reaction zone. a s

4. In a process. for producing K MnO by oxidizing manganese dioxide, in a hot aqueous potassium hydroxide solution, the improvement which :comprises adding said manganese dioxide-to said solution, 1 intimately mixing an oxygen containing gas with said solution to oxidize said manganese dioxide to K MnO therate of said manganese dioxide addition, being nbts'ubstantially greater '[than itsrate of oxidation to K MnO removing the K M nO produced, and oxidizing the K MnO removed to*K MnO in a separate reaction zone.

tic of the process is that the manganese oxide is added f ata rate which is not substantially greater than its rate of oxidation to K MnO When this important condition is not observed, andrthe manganese oxide is added subjstantially more rapidlythan it is oxidized, the excess oxide in the suspension swells up, prevents adequate'agitation and contact of oxygen with the mixture, so that the process is unworkable. The addition' of the manganese oxide according to the invention also takes advantage of the reaction rate and provides asuccessful continuous process. The. manganese dioxide is substantially consumed, sothat the K li I110, solution conveyed to .the second reaction zone for production of K MnO contains an insubstantial quantity ofmanganese dioxide, avoiding contamination-oi the K MnO4 product.

5. In a process for producing K MnO by oxidizing manganese dioxide in a hot aqueous potassium hydroxide solution, the improvement which comprises providing an aqueous about to 90% potassium hydroxide solution and maintaining it at a' temperature of about 240 C., to 300 (3., adding said manganese dioxide to said solution, intimately mixing an oxygen-containing gas with said solution to oxidize'said manganese dioxide to K MnO the rate of said manganese dioxide addition being not substantially greater than its rate, of oxidation to K MnO removing the K MnO produced, and oxidizing the K MnO removed to K MnO in an aqueous about 65 to 90% potassium hydroxide solution-maintained at a temperature of about-210 C. to 230 C. in a separate -reaction' zone.

'The process-of the invention is rapid and reliable, and

sumption of materials is a minimum,'and the operation is carried out in but a small equipment installation with low investment, power, heat, labor and maintenance requirements The invention provides a very advantageous solui produces high quality products in high yields. The con-. 7

6. The process forl K Mnogwhich comprises'adding a manganeseoxide having a valence of less than 5 to a hot aqueous about 65 %.to 90% potassium hydroxide solution at a temperature of aboutll C. to 350 Cgfintimatelyvmixing an oxygenrcontaining gas with the solution to-oxidizesaid manganese oxide to 14 mm,, the a rate ofsaid' manganese oxide addition being not substani y' greater than its rate o f oxidation to K MnO re= moving the produced, and oxidizing the K MnO removed to K MnQ in a separate reaction'zone by intimately mixing an oxygen-containing gas with an aqueous about 65% to 90% potassium hydroxide solution of the former at a temperature of about 140 C. to 310 C.

7. The process which comprises adding a manganese oxide having a manganese valence of less than to a hot aqueous potassium hydroxide solution, and intimately mixing an oxygen-containing gas with said solution to oxidize said manganese oxide to K MnO the average rate of said manganese oxide addition being a maximum of about A of a gram per liter of reaction mixture per minute and being not substantially greater than its rate of oxidation to K MnO 8. The process which comprises adding a manganese oxide having a manganese valence of less than 5 to a hot aqueous potassium hydroxide solution, intimately mixing an oxygen-containing gas with said solution to oxidize said manganese oxide to K MnO the average rate of said manganese oxide addition being a maximum of about of a gram per liter of reaction mixture per minute and being not substantially greater than its rate of oxidation to K MnO and removing the K MnO produced. v

9. The process which comprises adding a manganese oxide having a valence of less than 5 to a hot aqueous about 65% to 90% potassium hydroxide solution, maintaining said solution at a temperature of about 170 C. to 350 C., and intimately mixing an oxygen-containing gas with said solution to oxidize said manganese oxide to K MnO the average rate of said manganese oxide addition being a maximum of about of a gram per liter of reaction mixture per minute and being not substantially greater than its rate of oxidation to K MnO 10. The process which comprises adding manganese dioxide to a hot aqueous potassium hydroxide solution, and intimately mixing an oxygen-containing gas with said solution to oxidize said manganese dioxide to K MnO the average rate of said manganese dioxide addition being a maximum of about of a gram per liter of reaction mixture per minute and being not substantially greater than its rate of oxidation to K MnO 11. The process which comprises adding manganese dioxide to a hot aqueous about to potassium hydroxide solution, maintaining the temperature of said solution at about C. to 350 C., and intimately mixing an oxygen-containing gas with said solution to oxidize said manganese dioxide to K MnO the average rate of said manganese dioxide addition being a maximum of about of a gram per liter of reaction mixture per minute and being not substantially greater than its rate of oxidation to K MnO Scholder et al.: Zeitschrift fiir Anorganische und Allgemeine Chemie, 1954, Band 277, pages 243--2 45. 

1. IN A PROCESS FOR PRODUCING K2MNO4 BY OXIDIZING A MANGANESE OXIDE HAVING A MANGANESE VALENCE OF LESS THAN 5 IN A HOT AQUEOUS POTASSIUM HYDROXIDE SOLUTION, THE IMPROVEMENT WHICH COMPRISES ADDING SAID MANGANESE OXIDE TO SAID SOLUTION AND OXIDIZING IT THEREIN TO K3MNO4, THE AVERAGE RATE OF SAID MANGANESE OXIDE ADDITION BEING A MAXIMUM OF ABOUT 3/4 OF A GRAM PER LITER OF REACTION MIXTURE PER MINUTE AND BEING NOT SUBSTANTIALLY GREATER THAN ITS RATE OXIDATION TO K3MNO4, AND OXIDIZING THE K3MNO4 PRODUCED TO K2MNO4. 